Email already exists error in a upsert_by_email call

[cyrilchampier]

Exception:
Pardot::ResponseError: A prospect with the specified email address already exists

Stacktrace:
[GEM_ROOT]/bundler/gems/ruby-pardot-837bae6ead90/lib/pardot/http.rb:56 in check_response
[GEM_ROOT]/bundler/gems/ruby-pardot-837bae6ead90/lib/pardot/http.rb:19 in post
[GEM_ROOT]/bundler/gems/ruby-pardot-837bae6ead90/lib/pardot/objects/prospects.rb:65 in post
[GEM_ROOT]/bundler/gems/ruby-pardot-837bae6ead90/lib/pardot/objects/prospects.rb:50 in upsert_by_email

Appart from a bug in Pardot, I canot see how it can happen !?